Trackers refer to the campaigns you create for your hashtags and keywords. They look like this:
I.e. 'Catsunday' is the tracker I have created to track the keyword 'catsunday'
Whereas for posts, it is referring to the posts we have collected, within the tracker:
Here is an example of 4 posts collected under 'Catsunday' tracker.
The relationship between a tracker and posts is that posts are collected based on the keywords you set up for the tracker. You will have a limited amount of trackers to create within your account and a certain amount of posts you can track based on your subscription.
For example, a Professional plan only allows you to create 3 hashtag trackers and collect 20,000 posts per month whereas a Corporate plan allows you to create 10 hashtag trackers and collect 150,000 posts per month.
Find out what happens when you have reached your quota limit.